The present invention concerns induction type watthour meters for measuring alternating current electrical energy consumed by a load. In particular, the invention concerns watthour meters utilizing an overload compensation to achieve accurate readings at higher power loads.
A typical induction watthour meter consists of a rotor with integral conductive disk and a stator which induces currents in the conductive disk to which it is magnetically coupled. The stator consists of a potential stator energized by the line voltage and a current stator energized by load current. Torque is developed in the meter's conductive disk as a function of the magnitude of the product of the eddy currents induced in the disk by magnetic flux generated by both the potential and current stators. The developed torque is also proportional to the sine of the electrical phase angle between the two eddy current inducing magnetic fluxes.
While the watthour meter is typically very accurate over a wide range of loads, there must be compensation for a variety of factors which affect the accuracy of the meter. These compensations are built into the meter and provide corrections needed to make the meter register accurately under variations of load, temperature, frequency, and voltage. In particular, this invention relates to overload compensation in an induction watthour meter. A watthour meter must have overload compensation to integrate accurately at loads up to the maximum load as established by its class range.
The major factor requiring overload compensation is the increased damping of the disk caused by the increasingly significant magnitude of current flux at overloads. Thus at class load, the current flux damping may approach or exceed 10% of the total drag on the disk, causing the meter to have a negative error approaching 10% or greater. The static torque of the non-overload compensated meter is essentially linear throughout the range of overload currents. However, damping or drag at high currents (overload) causes the negative registration error of the meter to increase at overload.
To compensate for the increasing registration error with increasing loads in an uncompensated meter on overload, a magnetic shunt is held in place adjacent, but not touching, the poles of the current core of the current stator by a non-magnetic bracket. This shunt has little effect on overload compensation below the load at which the accuracy curve of the meter would otherwise start to drop. But, as the load increases, the shunt approaches saturation causing the current flux which cuts the disk to increase at a greater rate than the current.
Previous methods for providing the appropriate overload compensation of the watthour meter have relied on placing the magnetic shunt a pre-set distance from the poles of the current core. This set distance is known as the air gap and is determined experimentally to provide, on average, the appropriate overload compensation for the watthour meter. Once the air gap is established which provides the desired overload compensation for a class of meters, all meters within that class will have the same air gap, within the tolerances of the placement mechanisms.
However, placing the magnetic shunt a set distance from the poles of the current core has several drawbacks which the present invention improves upon or eliminates. Mechanically setting the distance of the air gap between the magnetic shunt and current core does not always provide the correct overload compensation. Mechanically setting the distance does not account for the effects of such factors as paint thickness, material thickness, stamping variations such as edge burr, roll-over, shear, and break area on the magnetic reluctance of the overload compensator air gap. Also, the ability to consistently hold a very small air gap to within the desired tolerance is not considered very practical in large volume production without the use of expensive close tolerance non-magnetic spacers. Setting of the overload compensator air gap as described herein compensates for minor variations that may occur in overload compensator magnetics or the magnetic induction at which the overload compensator approaches magnetic saturation.